1. Field of the Invention
This invention relates, in general, to methods for the manufacturing of test strips and, in particular, to methods for selectively combining multiple membranes for assembly into test strips.
2. Description of the Related Art
Various test strips have been developed for measuring the concentration of certain analytes in fluids and/or chemical properties of a fluid (e.g., pH or alkalinity). Such test strips can be used to measure, for example, glucose, cholesterol, proteins, ketones, phenylalanine or enzymes in blood, urine or saliva. These test strips frequently include multiple membranes that facilitate the determination of the analyte concentration or chemical property. For example, U.S. Pat. No. 6,162,397, which is fully incorporated herein by reference, describes a visual blood glucose test strip with two side-by-side membranes (i.e., paired membranes). Such paired membranes contain reagents which react with blood glucose to form visibly different colors (see also, Sherwood, M. et al., A New Reagent Strip (Visidex™) for Determination of Glucose in Whole Blood, Clinical Chemistry, 438–446 [1983]). A user can subsequently compare the two colors thus formed to a calibrated color chart (e.g., a color chart that includes sets of paired color pads) to ascertain blood glucose concentration.
FIG. 1 is a top plan view of a conventional visual blood glucose test strip 10. FIG. 2 depicts an exemplary calibrated color chart 200 for use with visual blood glucose test strip 10. Visual blood glucose test strip 10 includes a spreading top layer 12, an intermediate layer 14 with two membranes 14a and 14b (i.e., paired membranes 14a and 14b), and a support layer 16 with openings 16a and 16b. In operation, a user applies a blood sample to spreading top layer 12. As the blood sample penetrates spreading top layer 12, the blood sample spreads out and is substantially and uniformly distributed to paired membranes 14a and 14b. Glucose in the blood sample reacts with reagents in the paired membranes 14a, 14b, as it passes toward support layer 16, to form visually different colors in each of the paired membranes. The colors are viewed through openings 16a and 16b and compared with the paired color pads 202a–202h of calibrated color chart 200 to determine the blood glucose concentration of the blood sample. For the purpose of explanation only, calibrated color chart 200 in FIG. 2 is depicted to include eight sets of paired color pads (202a through 202h), each corresponding to one of eight targeted blood glucose test levels (e.g., 25, 50, 80, 120, 180, 240, 400 and 600 mg/dL). A user obtains a result by visually matching the paired membranes of a reacted visual blood glucose test strip to a paired set of color pads on calibrated color chart 200.
For quality assurance purposes during manufacturing, each lot of test strips with multiple membranes will customarily undergo acceptance testing in order to verify the accuracy of results obtained therewith. Such acceptance testing typically relies on any of a variety of standard color definition systems that specify color parameters for individual colors (for example, one of the color systems defined by the Commission Internationale de l'Eclairage (CIE) including the systems based on the L*a*b*color space and L*C*h color space). Methods for such acceptance testing are described in co-pending U.S. patent application Ser. No. 10/177,820 (tentatively identified by Attorney's Docket No. LFS-243 and incorporated herein by reference as if fully set forth) entitled “Acceptance Testing Method for Sets of Multiple Colored Workpieces.”
The acceptance testing of a lot of test strips is conventionally conducted after multiple membranes (each from a separate lot of membranes) have been combined and assembled into the lot of test strips. However, a particular combination of multiple membranes that has been assembled into a lot of test strips may not be optimal or even acceptable in terms of result accuracy. If a lot of test strips undergoing acceptance testing does not meet acceptance criteria for result accuracy, the entire lot of test strips is subject to rejection.
Still needed in the field, therefore, is a method for selectively combining multiple membranes for assembly into a test strip that minimizes test strip lot rejection. In addition, the method should be objective and yet account for user-related visual effects.